The multi-level modulation technique has so far been put to use especially in e.g., digital microwave communication. In particular, 2n QAM (quadrature amplitude modulation) system, where n is a natural number, exemplified by 4QAM, 16QAM, 32QAM, 64QAM, 128QAM and 256QAM, is preferentially used. Up to now, these modulation systems have been in use generally because of simplicity in circuitry. Recently, however, there is a strong demand for effective utilization of the frequency and the transmission power. Thus, it is strongly desired to achieve the required data transmission capacity without waste as the frequency bandwidth to be secured and as the digital microwave communication network is kept to as small a value as possible, in order to combat such exemplary situation in which, in case the 32QAM system is adopted to realize the data transmission capacity that is not possible to achieve with the 16QAM system, there is sufficient allowance in the data transmission capacity with the 32QAM system, such that the frequency bandwidth is taken up wastefully.